1. Field
The present disclosure relates to devices for extruding plastics, notably mixtures of elastomers which are intended to be used in the manufacture of tires.
2. Description of Related Art
These extrusion devices generally comprise an extrusion screw positioned inside a barrel and the purpose of which is to propel and pressurize the mixtures that are to be extruded, the final shape of the extruded product being given by an exit die. The screw also has the function of homogenizing the mixture and, in particular, of ensuring good uniformity of the thermal, rheological and chemical characteristics within the volume of the mixture, so that these remain constant at the exit die.
Various means intended to improve the homogeneity of the mixture in an extrusion device (or extruder) and which are positioned upstream of the exit die of the device are known. These means include, as known from document DE3805849, those which call upon homogenizing fingers arranged at the periphery of the barrel of the extruder and the ends of which protrude into the extrusion chamber to ensure better shearing of the mixture.
There was then a desire to adapt the performance of extruders to suit the various mixtures used with a given screw and a given barrel. Thus, one solution was to mount the homogenizing fingers in an adjustable manner. Such solutions are described for example in documents FR 2193685 or FR 2582251. Thus, by altering the position of the fingers in the extrusion chamber according to the type of mixture contained therein a better quality of mixing can be obtained, but at the expense of an operation that is costly and lengthy when all the time needed to set up all the fingers is added up. Moreover, the prior art also discloses solutions that propose adjusting all the fingers simultaneously, but these are complicated and present problems of sealing between the fingers and the extrusion chamber.
Other known solutions in the state of the art call upon the use of screens or breaker plates mounted at the exit of the extruder, just before the exit die for the extruded product. U.S. Pat. No. 3,756,574 and EP 0381171 describe such devices which improve the homogeneity of the mixture and screen out impurities. These screens or breaker plates mounted before the exit die have the disadvantage of retaining mixture in the mixture passage orifices and the disadvantage of the difficulty of cleaning these orifices.
Document FR 2636560 is also known and describes a device for continuously modifying the operation of an extrusion machine using means that move the screw axially with respect to the barrel of the machine. Such a device makes the construction of the machine complicated and somewhat unreliable.
Also known, from U.S. Pat. No. 3,191,229, is a machine for continuously mixing plastics or rubber comprising a conical ring which, when moved axially, makes it possible to adjust the gap at the exit of the machine. Such an adjustment is, however, sensitive to the axial positioning of the screw.
Document GB 894 127 describes an extruder for plastic tubes comprising a homogenization zone defined between a homogenizing ring and the downstream end of the screw. However, the ring is secured to the tubular extrusion nozzle and designed for a given size and specific material of extruded tube.
Moreover, extrusion machines or devices generally comprise an extrusion screw the form of which is tubular so that it can be cooled when the inside of the screw is placed in communication with a hydraulic cooling circuit. For that, the front end of the screw is closed off by a circuit closure plug and the upstream end of the screw is provided with means of connection to the hydraulic cooling circuit of the machine. In this type of machine, removing and replacing the screw entail disconnecting and draining the hydraulic cooling circuit.
Such is the case with U.S. Pat. No. 2,224,212 which describes a briquette press for briquettes made of sawdust or wood chippings and which comprises a removable forming head. Removal of the head assumes that the cooling circuit has been drained in order to avoid leaks.